The present invention relates generally to antennas for satellites and more particularly, to a side-fed reflector antenna for a satellite which provides a steerable antenna beam for full Earth field-of-view coverage with little degradation in the beam quality over the scan range.
In satellite communications systems, the antenna architecture has been to attach the entire antenna, comprising a parabolically curved main reflector, a feed horn, and a subreflector, to a positioning mechanism, such as a gimbal which moves the entire antenna to position or scan the antenna beam over the earth. Two factors contribute to the heavy weight of such a system. First, to maneuver a large mass and therefore the momentum, a heavy duty gimbal system is necessary. Second, to secure the entire antenna assembly in place during the launching vibration requires the use of a heavy latching structure during launch.
One antenna that addresses the above concerns is described in U.S. Pat. No. 5,870,060 and is depicted in FIG. 1. The antenna has a fixed non-moving feed 3 and associated electronics 5 and, a gimbaled 7,9 main reflector 10. Only the reflector 10 is moved to scan the beam, depicted by the dotted lines and arrows marked 11. The shortfall of this antenna is that it incurs high scan losses which is compensates for by special design of the reflector 10 and feed 3, which is expensive. This antenna additionally utilizes a long focal length to minimize the scan loss which results in the antenna requiring a substantial amount of real estate on a spacecraft which is typically at a premium. The antenna also uses an oversized reflector 10 to compensate for the gain loss. These compensations however do not solve the high cross-polarization level, high sidelobe level, and beam distortion problems which occurs when the reflector 10 is scanned off axis, particularly when the antenna is scanned to high scan angles such as the +/xe2x88x9211 degrees required for earth coverage from a geosynchronous satellite. The long focal length additionally results in the antenna requiring a substantial amount of real estate on a spacecraft which is typically at a premium.
What is needed therefore is a light weight antenna which has a low cross-polarization level and low beam distortion when scanned over a field of view, particularly when scanned over the Earth from a geosynchronous orbiting satellite.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a steerable antenna. In a first aspect, the steerable antenna assembly comprises a main reflector, a feed and a subreflector which together are oriented to define a side-fed dual reflector geometry where the feed is to a side of both the subreflector and the main reflector. The feed, subreflector and main reflector together producing an antenna beam which is directed in a preselected direction by the main reflector. A gimbal is coupled to the main reflector for positioning the main reflector and scanning the antenna beam over a preselected coverage area. The feed and subreflector remain substantially fixed in position when the main reflector is positioned and the antenna beam is scanned.
In a second aspect, the steerable antenna is coupled to a satellite in a geosynchronous orbit about the earth where the earth subtends approximately a twenty two degree cone of coverage from the satellite. The main reflector and gimbal are configured to scan the antenna beam over the earth field of view.